In recent years, photographic light-sensitive materials having high sensitivity have been rapidly put on the market with advances in technology relating to photographic light-sensitive materials for photography. As the result of reaching high sensitivity in photographic materials, the useful range of photographing has expanded to, for example, the case of photographing subjects in a dark room without using an electronic flash, the case of photographing subjects using a telephoto lens at high shutter speeds, such as a sport photographs and the case of photographing subjects requiring long exposure times, such as an astronomical photography.
To increase the sensitivity of photographic light-sensitive materials, many investigations have been made on various techniques including, for example, varying the shape, composition, preparation method, chemical sensitization and spectral sensitization of silver halide grains, the additives used and the structure of the couplers used, and some useful inventions have been also made. However, the requirement for photographic light-sensitive materials of high sensitivity has surpassed the progress of technology and thus these known methods are not satisfactory. Therefore, it has been conventional in the art to prepare photographic materials having high sensitivity by increasing the size of silver halide grains in combination with another technique(s) in order to increase sensitivity.
When the size of silver halide grains used in the emulsion is increased, sensitivity can be raised to a certain extent. However, if the content of silver halide is maintained constant, this necessarily results in decrease in the number of silver halide grains in the emulsion and, therefore, decreases the number of development initiating points, whereby a severe problem is encountered with graininess. In order to overcome this problem, many approaches have been known, for example:
Using a photographic light-sensitive material having at least two emulsion layers having the same spectral sensitivity and different sensitivities, that is, using silver halide grains of different sizes as described in British Patent No. 923,045 and JP-B-49-15495 (the term "JP-B" as used herein means an "examined Japanese patent publication"):
Using a coupler having high reactivity as described in JP-A 55-62454 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"):
Employing a DIR coupler or DIR compound as described in U.S. Pat. Nos. 3,227,554 and 3,632,435:
Using a coupler which forms a mobile dye as described in British Patent No. 2,083,640: and
Using a silver halide emulsion having a high average silver iodide content as described in JP-A-60-128443. Although these methods represent excellent technologies which exhibit substantial effects, they are still insufficient to satisfy the current strict requirements for both high sensitivity and high image quality.
Accordingly, in order to increase the size of silver halide grains and simultaneously increase the number of development initiating points as much as possible, it has been suggested to increase the content of silver halide grains in color negative photographic materials of high sensitivity while maintaining other properties such as the desilvering property in a bleach-fixing processing.
Another technique for increasing sensitivity/graininess ratio is to utilize tabular silver halide grains as described in JP-A-58-108525, JP-A-58-111935, JP-A-58-111936, JP-A-58-111937, JP-A-58-113927 and JP-A-59-99433. Tabular silver halide grains are very advantageous for improvement of the sensitivity and size ratio of silver halide grains since they have a larger surface area than grains of other known forms such as cubic, octahedral, tetradecahedral, and "lump" forms at the same volume. This allows higher adsorption of sensitizing dye onto the grain surface. Considering that most of the currently employed silver halide photographic materials use a silver halide emulsion spectrally sensitized with a sensitizing dye absorbed thereon, the use of the tabular silver halide grains is highly effective to improve the sensitivity and image quality of silver halide photographic materials for practical use.
Silver halide employed in such photographic light-sensitive materials is normally subjected to chemical sensitization in order to obtain the desired sensitivity and gradation.
For the chemical sensitization of silver halide, it is known that sulfur sensitization in which compounds containing sulfur capable of reacting with a silver ion and active gelatin can be used, reduction sensitization in which reducing compounds are used and noble metal sensitization in which gold and other noble metal compounds are used, can be successfully employed, either singly or in combination.
Sulfur sensitizers which can be used include thiosulfates, thioureas, thiazoles, and rhodanines. Specific examples of these compounds are described in U.S. Pat. Nos. 1,574,944, 2,410,689, 2,278,947, 2,728,668, 3,656,955, 4,030,928 and 4,067,740.
Reduction sensitizers which can be used include stannous salts, amines, hydrazine derivatives, form-amidinesulfinic acid, and silane compounds. Specific examples of these compounds are described in U.S. Pat. Nos. 2,487,850, 2,419,974, 2,518,698, 2,983,609, 2,983,610, 2,694,637, 3,930,867 and 4,054,458.
For noble metal sensitization, in addition to gold complex salts, complex salts of Group VIII metals of the Periodic Table, e.g., platinum, iridium, and palladium, can be used. Specific examples of these compounds are described in U.S. Pat. Nos. 2,399,083 and 2,448,060, and British Patent No. 618,061.
Recently, it has been highly desired to increase the sensitivity of silver halide emulsions. In order to achieve this purpose, it is ordinarily necessary to use a sensitization method where a sulfur sensitizer and a noble metal sensitizer are used in combination, most particularly a gold and sulfur sensitization method where a sulfur sensitizer and a gold sensitizer are employed in combination from among the chemical sensitization methods described above.
In such a case, the amounts of these sensitizers to be added are determined depending on various factors including the state of silver halide crystal grains to be chemically sensitized (for example, the grain size, grain size distribution, halogen composition, and crystal habit), environmental conditions (for example, the amount and type of binder used, the pH, pAg, reaction temperature and reaction time), assistants for gold sensitization (for example, accelerators typically represented by thiocyanates or thioether compounds, and antifoggants typically represented by thiosulfonates), as well as the types of sulfur and gold sensitizer to be employed.
However, it has been found that photographic light-sensitive materials of high sensitivity thus prepared have problems. Specifically, these photographic materials are accompanied by a degradation of photographic performance such as an increase in fog, a reduction in sensitivity and a deterioration in graininess with the passage of time between the production thereof and use thereof.
It was previously found by one of the present inventors that a silver halide photographic material having high sensitivity and improved pressure resistance characteristics could be obtained using a tabular silver halide emulsion containing a dispersion medium and tabular silver halide grains having a diameter of at least 0.4 .mu.m and an average aspect ratio of at least 2 in a ratio of at least 50% based on the total projected area of all silver halide grains in the emulsion, the tabular silver halide grains have a substantially stratified structure containing at least one layer having planes parallel to the two main planes facing each other thereof, and the average halogen composition of each layer in the material differing from that of each layer adjacent thereto by at least 1 mol %. This approach is described in JP-A-63-106746.
However, the effects obtained by the use of a combination with such a structure and a gold sensitization method according to the present invention could not be expected.